Method for manufacturing a multi-domain liquid crystal cell

ABSTRACT

A method for fabricating a multi-domain liquid crystal cell is disclosed, wherein first and second alignment directions are formed in first and second portions of an alignment layer provided on a substrate by selectively subjecting the first and second portions to different energy doses of linearly polarized ultraviolet light. Liquid crystal material is then injected between the one substrate and another substrate and into contact with the alignment layer, thereby obtaining a wide viewing angle in the liquid crystal device.

BACKGROUND OF THE INVENTION

[0001] The present invention relates to a method for manufacturing amulti-domain liquid crystal cell, and more particularly to a method formanufacturing a multi-domain liquid crystal cell in which the liquidcrystal director is aligned by irradiating an alignment layer withlight.

[0002] Liquid crystal displays, or LCDs, generally include twotransparent substrates with liquid crystal material injectedtherebetween. The liquid crystal (LC) material typically includesanisotropic molecules, the average direction of the long axes which arereferred to as the director of the LC material. The directordistribution in bulk LC material is determined by its azimuthalanchoring energy of the LC molecules on the substrates andcharacterized, in part, by the axes of easy orientation, whichcorrespond to the minimum surface energy of the LC material. Additionalparameters determining the director distribution include the pretiltangle between the director and the substrate plane.

[0003] In order to obtain uniform brightness and high contrast ratio ofthe LCD, the LC molecules must be appropriately aligned or homogeneouslyaligned after being injected between the substrates of the display.

[0004] Alignment of the LC molecules is achieved by providing analignment layer on the surface of the substrate. Preferably, thealignment layer includes a plurality of directional “domains” or regionshaving different alignment directions. If a plurality of binary domains,i.e., domains oriented in different directions, are provided on thesurface of the alignment layer, a uniform viewing angle can be achieved.Both the value of the director tilt and the direction of this tilt(i.e., direction of the axis of easy orientation) are important fornormal operation of LC devices having such binary, as well asmulti-domain structures.

[0005] The alignment layer is typically fabricated by depositing aspecially treated polymer on the surfaces of the substrates of thedisplay. In accordance with one conventional technique, homogenousalignment is achieved by subjecting the polymer to a rubbing process tomechanically form alignment microgrooves in the polymer layer. Theliquid crystal molecules are thus homogeneously or uniformly aligned dueto the intermolecular interaction between the polymer of the alignmentlayer and the liquid crystal molecules.

[0006] In the above described rubbing process, however, defects areformed in the microgrooves which cause light scattering and random phasedistortion. Moreover, dust and electrostatic discharges are produced inthe alignment layer, so that the substrate is damaged and yield isdecreased.

[0007] LC alignment by irradiation of photosensitive polymers withpolarized UV light has been proposed as an alternative to rubbing (M.Schadt et al., Jpn. J. Appl. Phys., 31 (1992). p. 2155; T. Marusii andYu. Reznikov et al., Mol., Master., 39,1993, p. 161). The aligningability of these photosensitive materials is determined by theiranisotropic photo-induced properties. In the present invention, thephotoalignment process is applied to create an array of domains wherethe easy orientation axes can possess two possible orthogonaldirections.

[0008] Materials based on polyvinyl cinnamate, polysiloxane andpolyamide are the most common photoaligning materials for LC displays.The directions of the easy axes in the plane of an aligning materialwere reported to be usually perpendicular to UV light polarizeddirection.

[0009] Such alignment techniques have advantages over the conventionalrubbing method described above. In particular, electrostatic charges anddust are not produced on the aligning surface, as in the rubbingprocess. Further, by appropriate exposure of the photosensitive polymer,it is possible to control the direction of the easy orientation axis onthe aligning surface and the azimuthal anchoring energy value. Further,the prescribed director distribution in an LC cell can be created.

[0010] Photoalignment techniques can also be used to generate aplurality of binary domains or a binary multi-domain structure. In onesuch technique described in W. Gibbon et al. (Nature, 351 (1991), p.49), a first photosensitive substrate is rubbed unidirectionally,followed by irradiation of the substrate through a mask with polarizedlight to induce the easy axis perpendicular to the direction of rubbing.When the LC cell is assembled by injecting LC molecules between thefirst substrate and a second polymer-coated substrate which was rubbedin the same direction as the photosensitive material, the LC moleculesare oriented with a 90°-twisted in regions corresponding to thetransparent parts of the mask. Instead of a mask, an image formationoptical system in the plane of the substrate can be used. The maindrawback of this method is the necessity to use rubbing, which leads tothe accumulation of dust and electrostatic charge, as well as theformation of distorted microgrooves on the aligning surfaces.

[0011] In another technique described in P. Shenon et al. (Nature, 368(1994), p. 532), instead of rubbing the photoaligning surface, thephotoalignment layer is exposed with polarized light to impart on aninitial background alignment director. This method is free of thedrawbacks described above, but has its own disadvantages. Namely, thismethod requires a double exposure of light with orthogonal polarizationthat requires rearrangement of the apparatus used to perform the opticalexposure.

SUMMARY OF THE INVENTION

[0012] An object of the present invention is to provide a simple methodfor producing binary multi-domain directional alignment in an LC cell,which does not possess the drawbacks of the known methods. It is afurther object of the present invention to create binary multi-domaindirectors in an alignment layer without any rearrangement of the opticalscheme.

[0013] It has been discovered that the initial easy axis of the polymerfused in photoalignment techniques change sharply by 90° when theintensity or dose of incident light exceeds a particular threshold.

[0014] Thus, in accordance with the present invention a method forcontrolling the alignment direction is provided, comprising the steps ofcoating a substrate with an alignment layer of a photosensitivematerial; irradiating the alignment layer with a first energy dose oflight to impart a first alignment direction; irradiating the alignmentlayer with a second energy dose of light to impart a second alignmentdirection, the second alignment direction being perpendicular to thefirst alignment direction.

[0015] In addition, the method for fabricating a multi-domain LC cellusing the substrate made from the above method comprises the steps ofproviding a first substrate and a second substrate, the first substrateis covered with a first alignment layer and the second substrate iscovered with a second alignment layer; irradiating the first and secondalignment layers with light to impart different alignment directionsdepending upon the light energy dose absorbed in each domain; assemblinga cell from two substrates where the alignment layers face one another;and injecting LC material between the first and second substrates.Control of the energy dose absorbed in each domain can be achieved byvarying the radiation intensity or duration.

[0016] According to another aspect of the present invention, thephotosensitive material for the alignment layer comprises polymersillustrated in FIGS. 1-4.

[0017] The invention will be set forth in part by the detaileddescription that follows and, in part, will be made obvious from thisdescription, or may be learned by practice of the invention. Theobjectives and advantages of the invention will be realized and attainedby means of the actions action and their combinations pointed out in theappended claims.

[0018] The accompanying drawings, which are incorporated in andconstitute a part of this specification, illustrate an embodiment of theinvention and together with the description, serve to explain theprinciples of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

[0019]FIG. 1 shows the chemical structure of photoalignment materialPSCN-1 according to an embodiment of the present invention.

[0020]FIG. 2 shows the chemical structure of photoalignment materialPSCN-2 according to an embodiment of the present invention.

[0021]FIG. 3 shows the chemical structure of photoalignment materialPSCN-3 according to an embodiment of the present invention.

[0022]FIG. 4 shows the chemical structure of photoalignment materialPSCN-4 according to an embodiment of the present invention.

[0023]FIG. 5 shows a device for controlling alignment directionaccording to an embodiment of the present invention.

[0024]FIG. 6 shows a partial perspective view of FIG. 5.

[0025]FIG. 7 shows cross-sectional view illustrating a Two-Domain TNstructure invention.

[0026]FIG. 8 shows a graph illustrating relationship between thephoto-energy and alignment direction.

DETAILED DESCRIPTION OF THE INVENTION

[0027] It has been discovered that, in certain materials, the alignmentaxis can change depending on the intensity of incident light and/orduration thereof. For example, FIG. 8 illustrates the relationshipbetween the alignment direction (φ) and the energy density of theincident light for a material such as PSCN-1. As seen in FIG. 7, φ isapproximately zero for energy densities below range W. In range W.however, φ is indeterminate or undefined, and is some unstable angleother than zero, e.g., 90 degrees. Therefore, the direction of thealignment axis can vary in accordance with energy dose of incident light(D=I_(exp)×t_(exp)). For example, the direction of the orientation ofstandard LC molecules in contact with an alignment layer formed with amaterial, such as PSCN-1, can shift by 90° if the energy density ofincident light exceeds a predetermined value.

[0028] Specifically, the irradiation of PSCN-1 material (shown inFIG. 1) by polarized nonfiltered light emitted by an Hg lamp withintensity I_(exp)=2 mW/cm² at wavelength 250 nm, for an exposure timet_(exp)=5 min, results in a dose (D=0.6 J) creating an easy axis eparallel to the direction polarization of the light[,] E_(exp). Incontrast, for exposure time t_(exp)=t_(thr)>10 min (D_(thr)=1.6 J),however, the direction [e] becomes perpendicular to E_(exp). In theintermediate region, no stable alignment is found.

[0029] Instead of irradiation during t_(exp), one can change theintensity of light I_(exp) to obtain the same effect. Accordingly, forexample, PSCN-1 material can have a light-induced easy axis e parallelto E_(exp) at t_(exp)=5 min and I_(exp)=2 mW/cm². However, an orthogonaldirection can be obtained for the same t_(exp), i.e., t_(exp)=5 min, butwith I_(exp)=4 mW/cm².

[0030] Moreover, the exposure time needed to change the orthogonalposition can be effectively controlled by doping PSCN-1 with a materialcausing the PSCN-1 to be more susceptible to only one easy axisdirection. In addition, the exposure intensity is saved by doping PSCN-1with 10% by weight of the photoorientant PSCN-2, as shown in FIG. 2.herefore, having a stable easy axis perpendicular to E_(exp) can beobtained with half the threshold dose D_(thr) as that noted above. Thus,with an exposure energy density of 1 mW/cm² ₁ the above describedmixture of PSCN-1 can be exposed for 5 minutes to impart an alignmentdirection parallel to the polarization of the incident light, and for 10minutes to impart an alignment direction perpendicular to thepolarization of the incident light. The same effect was observed forother photoalignment direction perpendicular to the polarization of theincident light.

[0031] The same effect was observed for other photoalignment materials,PSCN-3, PSCN-4, the chemical structures of which are shown in FIGS. 3and 4, respectively.

[0032] In accordance with the present invention, these materials, andother such compounds, can thus be used to control the easy axesdirection on an alignment surface by changing the irradiation dose oflight to produce a binary multi-domain director orientation in an LCcell. Further, multi-domain LCDs can be readily created with wideviewing angle characteristics while reducing the number of photomasksused in the process, and without rearranging the optical scheme orexposure apparatus during domain fabrication. Moreover, the presentinvention can be used to manufacture high density optical informationstorage cells where information is encoded in accordance with the binarydirection of the easy axis.

[0033]FIG. 5 is a schematic diagram showing a device for controlling analignment direction according to the present invention. Substrate 60 iscovered with photosensitive material 50 preferably having an easy axisdirection for the LC molecules which can be shifted depending upon thedose of incident UV light (D). Photosensitive material 50 is irradiatingwith the UV light from an Hg-lamp 10 transmitted through a lens 20,polarizer 30, and photomask 40 positioned close to the substrate 60.

[0034] As shown in FIG. 6, Photomask 40 includes first regions having afirst transmissivity T₁, and second regions having a second transparencyT₂. The radiation dose transmitted through the first regions ofphotomask 40 is preferably smaller than the threshold D_(thr) (athreshold dose of light, above which the alignment direction isperpendicular to E_(exp)), but is enough to produce a first alignmentdirection parallel to E_(exp) in corresponding first portions ofphotosensitive mask 40 having transmissivity T₂ is larger than D_(thr).As a result, the first portions of layer 50 impart easy axes to the LCmolecules parallel to E _(exp), and the second portions impart easy axese perpendicular to E_(exp).

[0035] In accordance with a further embodiment of the present invention,the first and second portions of photosensitive layer 50 are produced bycontrolling exposure time to match the conditions required for producingorthogonal easy axes. That is, the substrate can be irradiated twicethrough a photomask having “dark” and “transparent” regions. In thefirst step, the entire photosensitive material layer 50 is illuminatedwithout a mask for a time necessary to establish the first alignmentdirection (E_(exp)). In the second step, using a mask, only portions ofphotosensitive layer 50 corresponding to “transparent” regions of themask are illuminated for a time necessary to shift the first alignmentdirection to a second alignment direction E _(exp), which isperpendicular to the first alignment direction. As a result, regions ofphotosensitive layer 50 not exposed during the second step have thefirst alignment direction parallel to E_(exp) while portions irradiatedduring the second step through transparent parts of the mask have thesecond alignment direction E _(exp) perpendicular to E_(exp).

[0036] The method according to the present invention can be used forinformation storage in an LC cell where optical information is recordedas a binary code by producing pixels with LC molecules oriented alongorthogonal directions.

[0037] In accordance with the present invention, a binary domain LCDwith wide viewing angle characteristics can be obtained. FIG. 7illustrates a schematic diagram of two-domain TN (twisted nematic)structure of this invention. Each domain corresponds to an asymmetricviewing angle characteristic, but the total viewing characteristic,which is the sum of the asymmetric viewing angle characteristic of eachdomain, has a symmetric viewing angle. Thus, the main viewing angle iscompensated.

[0038] The preferred embodiment of the present invention will now befurther described in reference to specific examples. It should beunderstood that these examples are intended to be illustrative only andthe present invention is not limited to the conditions and materialsnoted therein. Various modifications can be achieved within thetechnical scope of the present invention. For example, as a modificationof the proposed method, a scanned light beam can be used instead of theirradiation through a photomask. In which case, the intensity of thebeam can be varied in order to deliver an appropriate energy dose to thedesired portion of the photosensitive layer.

EXAMPLE 1

[0039] A solution polymer material PSCN-1 in a 1:1 mixture of1,2-dichloroethane and chlorobenzene was prepared. The concentration ofthe polymer was 10 g/l. A polymer film was then spin-coated onto asubstrate with a rotation speed of 2500 rev./min. The substrate coatedwith the polymer film was prebaked after centrifuging at a temperatureof 200° C. for 2 hours.

[0040] The substrates were then positioned in the set up depicted inFIG. 5. The Hg-lamp 10 served as a source of the UV-light and the totalpower of the UV light in the plane of the photomask was 2 mW at 250 nm.A photomask having a binary transparent pattern was provided. Eachsquare pattern of pixels of the mask had an area of 4 mm×4 mm. Theilluminated area of the photomask was 2 cm×3 cm. The transparency of the“transparent” region was 85%, while transparency of the“semi-transparent” region was 30%. The substrate was irradiated for 10minutes.

[0041] After irradiating and drying the substrates, the LC cell having agap of 50 micrometers was assembled by a commonly used sandwichtechnique. The cell was filled with LC material, ZLI 4801-000, at roomtemperature, and the orientation was measured with a polarizedmicroscope.

EXAMPLE 2

[0042] The process of the second example is identical to the first[,].except the photosensitive materials includes 20% PSCN-2 and 80% PSCN-1.The substrates were irradiated for 5 minutes with the same result as inthe first example.

EXAMPLE 3

[0043] The process of the third example is similar to the first, exceptPSCN-3 was used as the photosensitive material. The cell was filled atan evaluated temperature of 100° C. and the LC, ZLI4801-000, wasinjected while in an isotropic phase. The substrates were irradiated for16 minutes and yielded the same result as in the first example.

EXAMPLE 4

[0044] The process of the fourth embodiment is the same as the firstexample[,]. except the cell was filled as an elevated temperature of100° C. and the LC, ZLI4801-000, was injected in an isotropic phase. Thesubstrates were irradiated for 20 minutes, and the same result wasobtained as in the first example.

EXAMPLE 5

[0045] The substrates were first prepared as in the first example. Atfirst, the entire substrates were irradiated without a photomask for 5minutes. The substrates were then irradiated through a binary photomaskfor 10 minutes. The photomask has a pixel pattern having alternatingopaque and transparent regions , with each square pixel occupying anarea of 4 mm×4 mm, and illuminated area of the photomask was 2 cm×2 cm.The transmissivity of the “transparent” region was 98%, and thetransmissivity of the “dark” or opaque region was 1%. The photomask wasthen removed and the LC cell was assembled and filled with LC, ZLI4801-000, as described in the first example.

EXAMPLE 6

[0046] Two substrates were successively coated with a transparentelectrode layer and photoalignment material were prepared as in thefirst example. The substrates were irradiated through a photomask havinga checker board pattern of “semi-transparent (T=30%)” and “transparent(T=85%)” square regions, each with an area of 3 mm×3 mm. The substratewas irradiated for 15 minutes. The LC cell with a cell gap of 5 μm wasassembled with domain twist structures having appropriate directororientations. The cell was filled at an elevated temperature of 100° C.and the injected LC, ZLI 4801-000, was in an isotropic phase.

[0047] It will be apparent to those skilled in the art that variousmodifications and variations can be made in the method for manufacturinga liquid crystal display of the present invention without departing fromthe scope or spirit of the invention.

[0048] Other embodiments of the invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. It is intended that the specificationand examples be considered as exemplary only, with a true scope andspirit of the invention being indicated by the following claims.

What is claimed is:
 1. A method for controlling an alignment directionin LC cell, comprising the steps of: subjecting a first portion of aphotosensitive alignment layer formed on a substrate to a first energydose to impart a first alignment direction in said first portion of saidphotosensitive alignment layer; and subjecting a second portion of saidalignment layer to a second energy dose to impart a second alignmentdirection in said second portion of said photosensitive alignment layer.2. A method for controlling an alignment direction according to claim 1,wherein the second alignment direction is perpendicular to the firstalignment direction.
 3. A method for controlling an alignment directionaccording to claim 1, wherein the first energy dose is different fromthe second energy dose.
 4. A method for controlling an alignmentdirection according to claim 3, wherein said step of subjecting saidfirst portion of said alignment layer to said first energy dosecomprises the step of: exposing said first portion of said alignmentlayer to light for a first time period; and said step of subjecting saidsecond portion of said alignment layer to said second energy dosecomprises the step of: exposing said second portion of said alignmentlayer to said light for a second time period.
 5. A method forcontrolling an alignment direction according to claim 3, wherein saidstep of subjecting said first portion of said alignment layer to saidfirst energy dose comprises the step of: exposing said first portion ofsaid alignment layer to a first light intensity; and said step ofsubjecting said second portion of said alignment layer to said secondenergy dose comprises the step of: exposing said second portion of saidalignment layer to a second light intensity.
 6. A method for controllingan alignment direction according to claim 1, wherein said first dose ofenergy comprises linearly polarized ultraviolet light.
 7. A method forcontrolling an alignment direction according to claim 1, wherein saidsecond dose of energy comprises linearly polarized ultraviolet light. 8.A method for controlling an alignment direction according to claim 1,wherein said photosensitive alignment layer includes a material selectedfrom the group of PSCN-1, PSCN-2, PSCN-3 and PSCN-4.
 9. A method forfabricating a multi-domain LC cell, comprising the steps of: providing afirst substrate coated with a first alignment layer; subjecting a firstregion of said first alignment layer to a first energy dose of light toimpart a first alignment direction; subjecting a second region of saidfirst alignment layer to a second energy dose of light to impart asecond alignment direction; providing a second substrate coated with asecond alignment layer; positioning said first substrate adjacent saidsecond substrate such that said first and second alignment layers faceone another; and injecting LC material between said first and secondsubstrates.
 10. A method for fabricating a multi-domain LC cellaccording to claim 9, wherein the second alignment direction isperpendicular to the first alignment direction.
 11. A method forfabricating a multi-domain LC cell according to claim 9, wherein thefirst energy dose is different from the second energy dose.
 12. A methodfor fabricating a multi-domain LC cell according to claim 11, whereinsaid step of subjecting said first region of said alignment layer tosaid first energy dose comprises the step of exposing said first regionof said first alignment layer to light for a first time period; andwherein said step of subjecting said second region of said firstalignment layer to said second energy dose comprises the step ofexposing said second region of said first alignment layer to said lightfor a second time period.
 13. A method for fabricating a multi-domain LCcell according to claim 12, wherein said step of exposing said secondregion of said first alignment layer includes the steps of: providing aphotomask having regions of varying transmissivity adjacent said firstalignment layer; and transmitting said light through said photomask toconcurrently expose said first and second regions of said firstalignment layer.
 14. A method for fabricating a multi-domain LC cellaccording to claim 12 further comprising the steps of: exposing saidfirst and second regions of said first alignment layer for said firsttime period; disposing a photomask having a first portion having a firsttransmissivity aligned with said first region of said first alignmentlayer and a second portion having a second transmissivity aligned withsaid second region of said first alignment layer, said firsttransmissivity being less than said second transmissivity; exposing saidfirst alignment layer with light transmitted through said photomask fora third time period, a sum of said third time period and said first timeperiod being equal to said second time period.
 15. A method forfabricating a multi-domain LC cell according to claim 11, wherein saidstep of subjecting said the first region of said first alignment layerto said first energy dose comprises the step of exposing said firstregion of said first alignment layer to a first light intensity; andwherein said step of subjecting said second region of said firstalignment layer to said second energy dose comprises the step ofexposing said second region of said first alignment layer to a secondlight intensity.
 16. A method for fabricating a multi-domain LC cellaccording to claim 15, wherein said step of exposing said second regionof said first alignment layer includes: providing a photomask havingportions of varying transmissivity adjacent said first alignment layer;and transmitting said light through said photomask.
 17. A method forfabricating a multi-domain LC cell according to claim 15, furthercomprising the steps of: disposing a photomask having a first portionhaving a first transmissivity aligned with said first region of saidfirst alignment layer and a second portion having a secondtransmissivity aligned with said second region of said first alignmentlayer, said first transmissivity being different than said secondtransmissivity; and exposing said first alignment layer through saidphotomask such that light having said first intensity impinges on saidfirst region of said first alignment layer and light having said secondintensity impinges on said second region of said first alignment layer.18. A method for fabricating a multi-domain LC cell according to claim9, wherein said second energy dose comprises linearly polarizedultraviolet light.
 19. A method for fabricating a multi-domain LC cellaccording to claim 9, wherein said photosensitive alignment layer,including a material selected from the group of PSCN-1, PSCN-2, PSCN-3and PSCN-4.
 20. A method for storing information in a photosensitivelayer comprising the steps of: exposing a first portion of saidphotosensitive layer to a first dose of radiation corresponding to afirst information; and exposing a second portion of said photosensitivelayer to a second dose of radiation different from said first dose andcorresponding to a second information.
 21. A method according to claim20, wherein said step of exposing said first portion of saidphotosensitive layer impart a first orientation in said first portion ofsaid photosensitive layer, and said step of exposing said second portionof said photosensitive layer, said method further comprising the stepof: bringing liquid crystal material in contact with said first andsecond portions of said photosensitive material, first molecules of saidliquid crystal material in contact with said first portion of said firstinformation, and second molecules of said liquid crystal material incontact with said second portion of said photosensitive material beingarranged in a second direction corresponding to said second information.22. A method according to claim 20, wherein said information is includesbinary information.
 23. A method according to claim 20 furthercomprising the steps of: directing a beam of energy to said firstportion of said photosensitive alignment layer to deliver said firstenergy dose; directing said beam of energy to said second portion ofsaid photosensitive alignment layer; and varying said beam of energy todeliver said second energy dose.
 24. A method for controlling analignment direction in LC cell, comprising the steps of: subjecting aphotosensitive alignment layer formed on a substrate to a first energydose to impart a first alignment direction; and subjection saidphotosensitive alignment layer to a second energy dose to shift thefirst alignment direction to a second alignment direction.
 25. Themethod for controlling an alignment direction according to claim 24,wherein said first energy dose is different than said second energydose.
 26. The method for controlling an alignment direction according toclaim 24, wherein said first alignment direction is substantiallyperpendicular to said second alignment direction.
 27. The method forcontrolling an alignment direction according to claim 24, wherein saidenergy dose includes a linearly polarized ultraviolet light.
 28. Themethod for controlling an alignment direction according to claim 24,wherein said photosensitive alignment layer includes a material selectedfrom the group of PSCN-1, PSCN-2, PSCN-3 and PSCN-4.